The present invention relates to steam turbine buckets, and in particular, to an improved steam turbine bucket flowpath design that eliminates flowpath steps between adjacent buckets.
In a steam turbine (not shown), the turbine buckets are typically installed adjacent to one another circumferentially around a turbine wheel (also not shown). FIG. 1a shows a stage 10 that includes a plurality of prior art buckets 12 as they would be mounted adjacent to one another around a portion of a turbine wheel. Each bucket 12 includes a dovetail 14 for mounting the buckets around the turbine wheel. To the outside of each dovetail 14 is a parallelogram-shaped platform 16 from which protrudes an airfoil 18. Each airfoil 18 includes a pressure side 20 that is partially concave in shape and a suction side 22 that is convex in shape. To the outside of each airfoil 18 is a parallelogram-shaped shroud 24.
Each bucket 12""s platform 16 includes a flat inner flowpath surface 26 at which one end 28 of the corresponding airfoil 18 intersects with platform 16. Similarly, each bucket 12""s shroud 24 includes a second flat outer flowpath surface 30 (FIG. 1c) at which the other end 32 of the corresponding airfoil 18 intersects with shroud 24.
Adjacent buckets, such as buckets 12a and 12b shown in FIG. 1b, form an inner flowpath 34, which is established by adjacent inner flowpath surfaces 26a and 26b of adjacent platforms 16a and 16b, by way of example. Adjacent buckets also form an outer flowpath 36, which is established by adjacent outer flowpath surfaces 30a and 30b of adjacent shrouds 24a and 24b of buckets 12a and 12b, again by way of example.
When buckets 12 are installed circumferentially around a turbine wheel, their airfoils 18 are each skewed with respect to the centerline (not shown) of the turbine. As they follow the circumference of the turbine wheel, airfoils 18 are staggered with respect to the centerline of the turbine. Each bucket 12""s platform 16 is then angled with respect to the centerline of the turbine to follow the stagger angle of the airfoils 18 around the turbine wheel. As such, as shown in FIG. 1b, flowpath steps 38 in flowpath 34 are created between the platforms 16 of adjacent buckets, such as platforms 16a and 16b of buckets 12a and 12b, shown by way of example in FIG. 1b. 
Similarly, each bucket 12""s shroud 24 is angled with respect to the centerline of the turbine to, again, follow the stagger angle of the airfoils 18 around the turbine wheel. Here again, as shown in FIG. 1c, flowpath steps 40 are created in an outer flowpath 36 between the shrouds 24 of adjacent buckets, such as shrouds 24a and 24b of buckets 12a and 12b, again by way of example.
FIGS. 1b and 1c depict typical distinct steps in the flowpaths of prior art steam turbine buckets. These circumferential steps in a turbine""s flowpath reduce the aerodynamic performance of the turbine. Advantageously, however, these prior art buckets can be manufactured using 3-axis milling machines that are not too expensive to buy or operate. In addition, manufacturing shops typically have 3-axis machines.
It should be noted that steam turbine buckets without flowpath steps can be made. Such buckets have inner and outer flowpath surfaces that are xe2x80x9csurfaces of revolutionxe2x80x9d about the centerline of the turbine. However, these buckets can be made only with 5-axis milling machines that are much more expensive to buy and operate than 3-axis machines. Thus, it would be very desirable for a manufacturer of steam turbines to have the ability to make buckets with 3-axis milling machines and that do not form flowpath steps when mounted together around a turbine wheel.
In an exemplary embodiment of the invention, a turbine bucket comprises a platform, a shroud, and an airfoil connected between the platform and the shroud, the platform including an inner flowpath surface at which the airfoil is connected to the platform, the inner flowpath surface being formed from a first inclined plane and a second inclined plane that meet at a first common boundary and form a first ridge or valley, the shroud including an outer flowpath surface at which the airfoil is connected to the shroud, the outer flowpath surface being formed from a third inclined plane and a inclined plane that meet at a second common boundary and form a second ridge or valley.
In another exemplary embodiment of the invention, a turbine bucket comprises a dovetail for mounting the bucket within the turbine, a platform connected to the dovetail, a shroud, and an airfoil connected between the platform and the shroud, the platform including an inner flowpath surface at which the airfoil is connected to the platform, the inner flowpath surface being formed from a first inclined plane and a second inclined plane that meet at a first common boundary and form a first ridge or valley, the shroud including an outer flowpath surface at which the airfoil is connected to the shroud, the outer flowpath surface being formed from a third inclined plane and a inclined plane that meet at a second common boundary and form a second ridge or valley, an inner flowpath being formed without steps by inner flowpath surface when it is positioned adjacent to a second inner flowpath surface of a second platform of a second bucket positioned adjacent to the bucket, and an outer flowpath being formed without steps by outer flowpath surface when it is positioned adjacent to a second outer flowpath surface adjacent of a second shroud of the second bucket positioned adjacent to the bucket.
In yet another exemplary embodiment of the invention, a turbine bucket comprises a dovetail for mounting the bucket within the turbine, a parallelogram-shaped platform connected to the dovetail, a parallelogram-shaped shroud, and an airfoil connected at a first end to the platform and at a second end to the shroud, the platform including an inner flowpath surface at which the first end of the airfoil intersects the platform, the inner flowpath surface being parallelogram-shaped and formed from a first triangular-shaped inclined plane and a second triangular-shaped inclined plane that meet at a first common boundary and form a first ridge or valley, the first common boundary bisecting the inner flowpath surface, the shroud including an outer flowpath surface at which the second end of the airfoil intersects the shroud, the outer flowpath surface being parallelogram-shaped and formed from a third triangular-shaped inclined plane and a fourth triangular-shaped inclined plane that meet at a second common boundary and form a second ridge or valley, the second common boundary bisecting the outer flowpath surface.
In a further exemplary embodiment of the invention, a stage of turbine buckets includes at least two buckets, each bucket comprises a dovetail for mounting the bucket within the turbine, a parallelogram-shaped platform connected to the dovetail, a parallelogram-shaped shroud, and an airfoil connected at a first end to the platform and at a second end to the shroud, the platform including an inner flowpath surface at which the first end of the airfoil intersects the platform, the inner flowpath surface being parallelogram-shaped and formed from a first triangular-shaped inclined plane and a second triangular-shaped inclined plane that meet at a first common boundary and form a first ridge or valley, the first common boundary bisecting the inner flowpath surface, the shroud including an outer flowpath surface at which the second end of the airfoil intersects the shroud, the outer flowpath surface being parallelogram-shaped and formed from a third triangular-shaped inclined plane and a fourth triangular-shaped inclined plane that meet at a second common boundary and form a second ridge or valley, the second common boundary bisecting the outer flowpath surface, an inner flowpath being formed without steps by adjacent inner flowpath surfaces when two of the plurality of platforms are positioned adjacent to one another, and an outer flowpath being formed without steps by adjacent outer flowpath surfaces when the two platforms are positioned adjacent to one another.
In yet a further exemplary embodiment of the invention, a turbine bucket flowpath comprises a first flowpath surface formed on a first platform or shroud of a first bucket, the flowpath surface being formed from a first inclined plane and a second inclined plane that meet at a first common boundary and form a first ridge or valley, the first common boundary bisecting the flowpath surface, and a second flowpath surface formed on a second platform or shroud of a second bucket, the second flowpath surface being formed from a third inclined plane and a fourth inclined plane that meet at a second common boundary and form a second ridge or valley, the second common boundary bisecting the second flowpath surface, the flowpath being formed by the second inclined plane of the bucket and the third inclined plane of the second bucket adjacent to the bucket, the second and third planes sharing a common flush edge at the interface between them.
In still a further exemplary embodiment of the invention, a inner turbine bucket flowpath comprises a first inner flowpath surface formed on a first parallelogram-shaped platform of a first bucket, the inner flowpath surface being formed from a first triangular-shaped inclined plane and a second triangular-shaped inclined plane that meet at a first common boundary and form a first ridge or valley, the first common boundary bisecting the inner flowpath surface, and a second inner flowpath surface formed on a second parallelogram-shaped platform of a second bucket, the inner flowpath surface being formed from a third triangular-shaped inclined plane and a fourth triangular-shaped inclined plane that meet and form a second ridge, which bisects the inner flowpath surface, the inner flowpath being formed by the second triangular-shaped inclined plane of the bucket and the first triangular-shaped inclined plane of the second bucket adjacent to the bucket, the first and second planes and sharing a common flush edge at the interface between them.
In still another exemplary embodiment of the invention, an outer turbine bucket flowpath comprises a first outer flowpath surface formed on a first parallelogram-shaped shroud of a first bucket, the outer flowpath surface being formed from a first triangular-shaped inclined plane and a second triangular-shaped inclined plane that meet at a first common boundary and form a first ridge or valley, the first common boundary bisecting the inner flowpath surface, and a second outer flowpath surface formed on a second parallelogram-shaped shroud of a second bucket, the outer flowpath surface being formed from a third triangular-shaped inclined plane and a fourth triangular-shaped inclined plane that meet at a second common boundary and form a second ridge or valley, the second common boundary bisecting the inner flowpath surface, the outer flowpath being formed by the second triangular-shaped inclined plane of the bucket and the third triangular-shaped inclined plane of the second bucket adjacent to the bucket, the second and third planes and sharing a common flush edge at the interface between them.